Galaxy simulation breaks new ground

Mar 31, 2006

Two astronomers have performed one of the world's largest astrophysics simulations to date in order to model the growth of galaxies. Using the "Earth Simulator" supercomputer in Japan, which is also used for climate modelling and simulating seismic activity, Masao Mori of the University of California at Los Angeles and Masayuki Umemura at the University of Tsukuba have calculated how galaxies evolved from just 300 million years after the Big bang to the present day. The results show that galaxies may have evolved much faster than currently believed (Nature440 644).

According to the "hierarchical" model, galaxies are formed via a bottom-up process that starts with the formation of small clumps of gas and stars that then merge into bigger systems. Mori and Umemura simulated this process using a powerful 3D hydrodynamic code combined with a "spectral synthesis" code for an astrophysical plasma in order to take into account the dynamical and chemical evolution of a primordial galaxy. The Earth-Simulator simulation was performed with an ultra-high resolution based on 1024 "grid points", making it one of the biggest calculations ever performed in astrophysics.

Mori and Masayuki set up the initial conditions in their simulation based on a cold dark matter universe, the parameters of which are determined by measurements of the cosmic microwave background. These observations, first made in 2003, show that we are living in a flat universe comprising just 4% ordinary matter, 22% dark matter and 74% dark energy -- in agreement with the standard model of cosmology. The researchers then directly compared their numerical results with observations of primitive galaxies called Lyman-alpha emitters and "Lyman break" galaxies, which astronomers find in the most distant and therefore oldest parts of the universe.

The results show that the primordial bubbles of gas that formed in the early universe just 300 millions years after the Big Bang do indeed look like Lyman-alpha emitters. After about 1 billion years, the simulations show that these galaxies mutate into Lyman break galaxies. Finally, after 10 billion years of evolution, the structures resemble present-day elliptical galaxies.

The simulation also predicts the mixture of chemical elements in the galaxy at each stage of its evolution, and suggests that our Milky Way has roughly the same composition today as it did when it was just 1 billion years old. Until now, galaxies were thought to have evolved gradually and become enriched in heavier elements beyond hydrogen and helium over a period of 10 billion years by repeated star formation and supernova explosions.

"Our finding shows that galaxy formation proceeded much faster and that a large amount of heavy elements were produced in galaxies in just 1 billion years," says Mori.